Windows: If you're in the habit of staying up way too late working on the computer, Bedtime Help serves as a gentle reminder and shut down scheduler to get you off to bed.
Bedtime Help is a free application you can configure to remind you ahead of your scheduled bedtime with popup and full screen reminders. In addition to giving you visual reminders Bedtime Help can also shut down your system at your schedule bed time or shortly there after.
You could recreate the effect of Bedtime Help using the built-in scheduler in Windows but it would require more effort and the individual pieces — notifications and shut down — wouldn't coordinate with each other. Bedtime Helper is freeware, Windows only. Have a tip, trick or application to help you unplug at the end of the day? Let's hear about it in the comments.